1. Field of the Invention
The present invention relates to a charged-particle apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions of an observed area on a sample surface. Hence, the apparatus can be used to inspect defects and/or particles on wafers/masks with high detection efficiency and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on surfaces of wafers/masks during fabrication processes, which reduce the yield to a great degree. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Accordingly, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beam employ the principle of scanning electron microscope (SEM) with a single electron beam, which therefore can provide higher resolution but can not provide throughputs competent for mass production. Although the higher and higher beam currents can be used to increase the throughputs, the superior spatial resolutions will be fundamentally deteriorated by Coulomb Effect.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots on one being-inspected or observed surface of a sample. For the sample surface, the plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≤50 eV) and backscattered electrons (energies close to landing energies of the electrons). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than scanning the large observed area with a single beam
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions by a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. The apparatus therefore is generally called as a multi-column apparatus. The plural columns can be either independent or share a multi-axis magnetic or electromagnetic-compound objective lens (such as U.S. Pat. No. 8,294,095). On the sample surface, the beam interval between two adjacent beams is usually as large as 30˜50 mm.
For the latter, a source-conversion unit is used to generate a plurality of parallel real or virtual images of the single electron source. Each image is formed by one part or beamlet of the primary electron beam generated by the single electron source, and therefore can be taken as one sub-source emitting the one beamlet. In this way, the single electron source is virtually changed into a plurality of sub-sources forming a real or virtual multi-source array. Within the source-conversion unit, the beamlet intervals are at micro meter level so as to make more beamlets available, and hence the source-conversion unit can be made by semiconductor manufacturing process or MEMS (Micro Electro-Mechanical Systems) process. Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of parallel images onto and scan the plurality of small scanned regions respectively, and one secondary projection imaging system focuses the plurality of secondary electron beams therefrom to be respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus with a plurality of beamlets therefore is generally called as a multi-beam apparatus and the conventional apparatus with a single electron beam is called as a single-beam apparatus.
Conventionally, the source-conversion unit comprises one image-forming means, and one beamlet-forming means or one beamlet-limit means. The image-forming means basically comprises a plurality of image-forming elements, and each image-forming element can be a round lens or a deflector. The beamlet-forming means and the beamlet-limit means are respectively above and below the image-forming means and have a plurality of beamlet-limit openings. In one source-conversion unit with one beamlet-forming means, at first the plurality of beamlet-limit openings divides the primary electron beam into a plurality of beamlets, and then the plurality of image-forming elements (round lenses or deflectors) focuses or deflects the plurality of beamlets to form the plurality of parallel real or virtual images. U.S. Pat. Nos. 7,244,949 and 6,943,349 respectively propose an multi-beam apparatus with one source-conversion unit of this type. In one source-conversion unit with one beamlet-limit means, at first the plurality of image-forming elements (deflectors) deflects a plurality of beamlets of the primary electron beam to form the plurality of parallel virtual images, and then the plurality of beam-limit openings cuts off peripheral electrons of the plurality of beamlets respectively. The first cross reference proposes a multi-beam apparatus 100A with one source-conversion unit of this type, as shown in FIG. 1.
In FIG. 1, for sake of simplification, the primary projection imaging system 130 is not shown in detail and the secondary projection imaging system and the electron detection device are not shown. The single electron source 101 on the primary optical axis 100_1 generates the primary electron beam 102 seemingly coming from the crossover 101s. The condenser lens 110 focuses the primary electron beam 102 and thereby forming an on-axis virtual image 101sv of the crossover 101s. The peripheral electrons of the primary electron beam 102 are cut off by the main opening of the main aperture plate 171. The source-conversion unit 120 comprises the image-forming means 122 with three image-forming elements 122_1, 122_2 and 122_3, and a beamlet-limit means 121 with three beam-limit openings 121_1, 121_2 and 121_3. Each image-forming element functions as one micro-deflector. The beam-limit opening 121_1 is aligned with the primary optical axis 100_1, and therefore the image-forming element 122_1 is not necessary to comprise one micro-deflector. The image-forming elements 122_2 and 122_3 respectively deflect beamlets 102_2 and 102_3 of the primary electron beam 102, and thereby forming two off-axis virtual images 102_2v and 102_3v of the crossover 101s. The deflected beamlets 102_2 and 102_3 are perpendicularly incident onto the beamlet-limit means 121. The beam-limit openings 121_1, 121_2 and 121_3 respectively cut off the peripheral electrons of the center beamlet 102_1 and the deflected beamlets 102_2 and 102_3, and thereby limiting the currents thereof. The focusing power of the condenser lens 110 varies the current density of the primary electron beam 102, and therefore is able to change the currents of the beamlets 102_1˜102_3. Consequently, three parallel virtual images 101sv, 102_2v and 102_3v form one virtual multi-source array low with variable currents. The primary projection imaging system 130 then images the virtual multi-source array 101v onto the being-observed surface 7 of the sample 8 and therefore form three probe spots 102_1s, 102_2s and 102_3s thereon. Each of the image-forming elements 122_1˜122_3 can have a dipole configuration (with two electrodes) which can generate one deflection field in its required deflection direction, or a quadrupole or 4-pole configuration (with four electrodes) which can generate one deflection field in any direction.
To compensate the inherent off-axis aberrations and the manufacturing derivative aberrations of the probe spots 102_1s, 102_2s and 102_3s, each of the image-forming elements 122_1, 122_2 and 122_3 can further function as one micro-compensator to compensate the field curvature aberration and the astigmatism aberration. Accordingly, each image-forming element can have a 4-pole lens (with four electrodes whose inner surfaces form a cylindrical surface) which can generate one deflection field in any direction, one quadrupole field in one specific direction and one round-lens field, or octupole or 8-pole lens (with eight electrodes whose inner surfaces form a cylindrical surface) which can generate one deflection field and one quadrupole field in any directions. The 4-pole lens needs to be oriented to make the specific direction of the quadrupole field match the direction of the astigmatism aberration. If a lot of beamlets is used, it may be difficult to manufacture a large number of 8-pole lenses or 4-pole lenses each with a specific orientation. In this case, for each beamlet, a pair of 4-pole lenses (such as 122_2dc-1 and 122_2dc-2) can be used in the way shown in FIGS. 2A˜2C. The upper and lower 4-pole lenses of one pair of 4-pole lenses are respectively placed in the upper and lower layers 122-1 and 122-2, aligned with each other and have a 45° difference in azimuth or orientation. For each image-forming element, the deflection field in any desired deflection direction and the round-lens field can be generated by either or both of the upper and lower 4-pole lenses, and the quadrupole field in any desired compensation direction can be generated by both of the upper and lower 4-pole lenses.
In FIG. 1 and FIG. 2A, each image-forming element performs the image-forming function and aberration-compensation function simultaneously. The performance of the aberration-compensation function with respect to a tilting beamlet will be inferior to a normal beamlet, and this will finally limit the available image resolutions of the apparatus. Accordingly, it is necessary to provide an apparatus of plural charged-particle beams, which can avoid the foregoing performance deterioration.